Numerical methods of star formation history measurement and applications to seven dwarf spheroidals

A comprehensive study of the measurement of star formation histories from colour-magnitude diagrams (CMDs) is presented, with an emphasis on a variety of subtle issues involved in the generation of model CMDs and maximum likelihood solution. Among these are the need for a complete sampling of the synthetic CMD, the use of proper statistics for dealing with Poisson-distributed data (and a demonstration of why Χ 2 must not be used), measuring full uncertainties in all reported parameters, quantifying the goodness-of-fit, and questions of binning the CMD and incorporating outside information. Several example star formation history measurements are given. Two examples involve synthetic data, in which the input and recovered parameters can be compared to locate possible flaws in the methodology (none were apparent) and measure the accuracy with which ages, metallicities and star formation rates can be recovered. Solutions of the histories of seven Galactic dwarf spheroidal companions (Carina, Draco, Leo I, Leo II, Sagittarius, Sculptor and Ursa Minor) illustrate the ability to measure star formation histories given a variety of conditions -numbers of stars, complexity of star formation history and amount of foreground contamination. Significant measurements of ancient >8 Gyr star formation are made in all seven galaxies. Sculptor, Draco and Ursa Minor appear entirely ancient, while the other systems show varying amounts of younger stars.

[1]  W. Freedman,et al.  The Star Formation History of the Local Group Dwarf Galaxy Leo I , 1998, astro-ph/9906121.

[2]  L. Girardi,et al.  Population effects on the red giant clump absolute magnitude, and distance determinations to nearby galaxies , 2001 .

[3]  D. Valls-Gabaud,et al.  Deriving star formation histories: inverting Hertzsprung‐‐Russell diagrams through a variational calculus maximum likelihood method , 1998, astro-ph/9809039.

[4]  William H. Press,et al.  Numerical Recipes in C, 2nd Edition , 1992 .

[5]  M. Mateo,et al.  The Star Formation History of the Carina Dwarf Galaxy , 1998, astro-ph/9804058.

[6]  D. Schlegel,et al.  Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds , 1997, astro-ph/9710327.

[7]  D. Astronomia,et al.  The Elusive Old Population of the Dwarf Spheroidal Galaxy Leo I , 1999, The Astrophysical journal.

[8]  K. Olsen.  Star Formation Histories from HST Color-Magnitude Diagrams of Six Fields of the LMC , 1999, astro-ph/9902031.

[9]  G. Costa,et al.  The age(s?) of the sculptor dwarf galaxy , 1983 .

[10]  Gisella Clementini,et al.  Distances, Ages, and Epoch of Formation of Globular Clusters , 1999, astro-ph/9902086.

[11]  Hubble Space Telescope observations of the Draco dwarf spheroidal galaxy , 1997, astro-ph/9709259.

[12]  Taft E. Armandroff,et al.  Standard Globular Cluster Giant Branches in the (M(I), (V - I)o) Plane , 1990 .

[13]  K. Olsen.  Star Formation Histories from Hubble Space Telescope Color-Magnitude Diagrams of Six Fields of the Large Magellanic Cloud , 1999 .

[14]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[15]  R. G. Gratton,et al.  Abundances for globular cluster giants: I. homogeneous metallicities for 24 clusters , 1996, astro-ph/9607078.

[16]  Kenneth J. Mighell,et al.  The Leo II Dwarf Spheroidal - an Old Galaxy With a Large Intermediate-age Population , 1996 .

[17]  A. Saha,et al.  Hubble Space Telescope Observations of the Local Group Dwarf Galaxy Leo I , 1998, astro-ph/9811122.

[18]  The Charge‐Transfer Efficiency and Calibration of WFPC2 , 2000, astro-ph/0006237.

[19]  K. Mighell.  WFPC2 Observations of the Carina Dwarf Spheroidal Galaxy. , 1997, astro-ph/9707237.

[20]  C. Burke,et al.  WFPC2 Observations of the Ursa Minor Dwarf Spheroidal Galaxy , 1998, astro-ph/9903065.

[21]  C. J. Grillmair,et al.  Observations and Implications of the Star Formation History of the Large Magellanic Cloud , 1999 .

[22]  The Sagittarius Dwarf Galaxy Survey (SDGS) — II. The stellar content and constraints on the star formation history , 1999, astro-ph/9903371.

[23]  A. Saha,et al.  The interpretation of color-magnitude diagrams through numerical simulation and Bayesian inference , 1996 .

[24]  E. Olszewski,et al.  The Ursa Minor dwarf galaxy : still an old stellar system. , 1985 .

[25]  William H. Press,et al.  Numerical recipes in C , 2002 .

[26]  Mario Mateo,et al.  Deep Hubble Space Telescope Imaging of IC 1613. I. Variable Stars and Distance , 2000, astro-ph/0012150.

[27]  Mario Mateo,et al.  DWARF GALAXIES OF THE LOCAL GROUP , 1998, astro-ph/9810070.

[28]  The 2MASS Color-Magnitude Diagram of the Center of the Sagittarius Dwarf Galaxy: Photometric Measurements of a Surprisingly High Mean Metallicity , 2001, astro-ph/0108061.

[29]  A. Bressan,et al.  The star formation history of the Large Magellanic Cloud , 1992 .

[30]  Gianpaolo Bertelli,et al.  The Local Group Dwarf Irregular Galaxy NGC 6822.II.The Old and Intermediate -Age Star Formation History , 1996 .

[31]  The Sagittarius Dwarf Galaxy Survey (SDGS) — I. Colour—magnitude diagrams, reddening and population gradients. First evidence of a very metal-poor population , 1998, astro-ph/9812344.

[32]  C. Alard.  Metallicity gradients in the Sagittarius dwarf spheroidal Galaxy , 2001, astro-ph/0108098.

[33]  Andrew E. Dolphin,et al.  WFPC2 Stellar Photometry with HSTphot , 2000, astro-ph/0006217.

[34]  A. Sarajedini,et al.  Technique for the Simultaneous Determination of Globular Clusters Metallicity and reddening Using (V,V-I) Color-Magnitude Diagrams , 1994 .

[35]  T. A. Smecker-Hane,et al.  The Stellar Population of the Carina Dwarf Spheroidal Galaxy. I. A New Color- Magnitude Diagram for the Giant and the Horizontal Branches , 1994 .

[36]  A. Dolphin.  A new method to determine star formation histories of nearby galaxies , 1997 .

[37]  M. Castellani,et al.  Stellar Populations in the Dwarf Spheroidal Galaxy Leo I , 1998, astro-ph/9812266.

[38]  E. Grebel,et al.  The stellar content and distance of UGC 4483 , 2000, astro-ph/0012471.

[39]  Antonio Aparicio,et al.  The Local Group Dwarf Irregular Galaxy NGC 6822.I.The Stellar Content , 1996 .

[40]  D. Zaritsky,et al.  A Method for Determining the Star Formation History of a Mixed Stellar Population , 2001, astro-ph/0104202.

[41]  L. Greggio,et al.  Star formation in dwarf irregular galaxies: preliminary results , 1989 .

[42]  A. Sarajedini,et al.  The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy , 1997, astro-ph/9706168.

[43]  L. Girardi,et al.  Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 , and from to 0.03 , 1999, astro-ph/9910164.